Phytosynthesis and characterization of tin-oxide nanoparticles (SnO2-NPs) from Croton macrostachyus leaf extract and its application under visible light photocatalytic activities

Nanotechnology is rapidly becoming more and more important in today's technological world as the need for industry increases with human well-being. In this study, we synthesized SnO2 nanoparticles (NPs) using an environmentally friendly method or green method from Croton macrostachyus leaf extract, leading to the transformation of UV absorbance to visible absorbance by reducing the band gap energy. The products underwent UV, FTIR, XRD, SEM, EDX, XPS, BET, and DLS for characterization. Characterization via UV–Vis spectroscopy confirmed the shift in absorbance towards the visible spectrum, indicating the potential for enhanced photocatalytic activity under visible light irradiation. The energy band gap for as-synthesized nanoparticles was 3.03 eV, 2.71 eV, 2.61 eV, and 2.41 eV for the 1:1, 1:2, 1:3, and 1:4 sample ratios, respectively. The average crystal size of 32.18 nm and very fine flakes with tiny agglomerate structures of nanoparticles was obtained. The photocatalytic activity of the green-synthesized SnO2 nanoparticles was explored under visible light irradiation for the degradation of rhodamine B (RhB) and methylene blue (MB), which were widespread fabric pollutants. It was finally confirmed that the prepared NPs were actively used for photocatalytic degradation. Our results suggest the promising application of these green-synthesized SnO2 NPs as efficient photocatalysts for environmental remediation with low energy consumption compared to other light-driven processes. The radical scavenging experiment proved that hydroxyl radicals (_OH) are the predominant species in the reaction kinetics of both pollutant dyes under visible light degradation.


Experimental procedure and materials used
All the chemicals (stannous tetrachloride pentahydrate (98%) (Molar mass (SnCl 4 , (M = 350.60)),Rhodamine B (RhB), Methylene blue (MB), isopropanol (IPA) and Na 2 EDTA were obtained from Sisco Research Laboratories Pvt. Ltd.In all the experiments, DI water and ethanol were used as a solvent.The chemicals were of analytical quality and employed without additional purification.Green synthesis of tin oxide nanoparticles from tin tetrachloride pentahydrate (SnCl 4 .5H 2 O) and plant extract involves a simple and environmentally friendly method.Here's a concise methodology: the Croton macrostachyus plant is chosen which is rich in bioactive compounds (phytochemicals) suitable for nanoparticle synthesis.The plant was extracted using water as a solvent.A tin salt precursor was used as the source of tin ions for NP formation.The plant extract is mixed with the tin salt solution in a controlled environment.The ratio between the plant extract and tin salt concentration can influence the size and stability of the nanoparticles.Heat is applied to maintain specific reaction conditions and to facilitate the reduction of tin ions by phytochemicals available in the plant extract.This reduction leads to the formation of tin oxide nanoparticles.To control the size and stability of the nanoparticles, reaction parameters such as temperature, pH, and reaction time were adjusted.The techniques of centrifugation and filtration are used to separate the synthesized tin oxide nanoparticles from the reaction mixture and washed to remove any impurities or unreacted precursors from the nanoparticles.The obtained nanoparticles were dried under controlled conditions to prevent agglomeration and ensure stability.The synthesized tin oxide nanoparticles were analyzed using characterization techniques like UV-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), EDX, and Fourier-transform infrared spectroscopy (FTIR) to confirm their size, structure, and composition.The potential applications of the green-synthesized tin oxide nanoparticles was explored and identified as used in visible light photocatalysis.The plant we have used in this report was cultivated in the local area of Nekemte Town, Oromia, Ethiopia.This study complies with relevant international, national, institutional and legislative guidelines.

Green synthesis of tin-oxide nanoparticles (G-SnO 2 NPs)
The 3 g of stannous tetrachloride pentahydrate (98%) was dissolved in 100 ml of DI water.To explore the effect of the extract on the properties of SnO 2 NPs different concentrations of samples were prepared as 1:1, 1:2, 1:3, 1:4, ratios of precursor to the extract following 10 ml:10 ml, 10 ml:20 ml, 10 ml:30 ml, 10 ml:40 ml respectively.Then, the solution was stirred at a temperature of 80 °C for 2 h at a revolution of 700 rpm.The solution was then sonicated for 30 min and cooled overnight.The solution was washed two times with ethanol and centrifuged for 20 min.The solution is dried for 2 h at 200 °C and calcined at 500 °C for 3 h.Subsequently, different concentrations of SnO 2 NPs were obtained which were used for UV-visible spectroscopy characterization, and for the whole characterizations and applications the 1:4 ratio is selectively used.

Photodegradation experiments
The degradation potentials of the prepared nanomaterials were studied through the decomposition of rhodamine B (RhB) and Methylene blue (MB) dyes.The solution was prepared for the rhodamine B (RhB) and Methylene Blue (MB) dyes by dissolving (0.1 g/L) of dyes powder in DI water.In this particular research, 50 mg of the metal oxide was treated with an aqueous solution of rhodamine B (RhB) and Methylene blue (MB) in DI water.Ultraviolet Visible (UV-Vis) spectra, observing the change in time of irradiation were occasionally recorded.Schematic diagram of Green synthesis route in SnO2 NPs are depicted in Fig. 1.

UV-Visible spectroscopic studies
The UV-Visible spectroscopy was taken in the wavelength range of 200-800 nm, for the different samples prepared with different ratios of precursor to extract as 1:1, 1:2, 1:3, 1:4 for SnO 2 NPs. Figure 2a,b shows the maximum absorption peaks of the as-synthesized SnO 2 NPs for different concentrations and their corresponding Tauc's plot.From the result one can conclude that as the concentration of the extract increases the properties of the catalyst changes due to electron transfer from the reducing and capping agent present in extract to the precursor and bioreduction happens which further affects the band gap of the synthesized nanoparticles.The band gap (E g ) of the material, is one of the optical features that demonstrate the semiconductor character of NPs.The optical property of the nanoparticles is obtained from TAUC's relation below 18 : ere α , h , ν , E g , and K are the absorption coefficient, Planck constant, light frequency, band gap energy, and TAUC constant, respectively.
As shown in Fig. 2b below, TAUC's figure has been used to determine the band gap of semiconductor nanometallic oxides.TAUC numbers are obtained by plotting (αhν) 2 vs hν(energy) 17 1) which shows different functional groups present in extract contribute to the electron transfer, to decrease the energy band gap and this report is agreed with previous report in which the band gap is decreased without any dopant by functional groups present in plant extracts [17][18][19] .When irradiated with visible light, these band gap values indicate the materials' potential for use in visible light spectrum photocatalysis 20 .The drop in energy gap value of 3.56 eV may also be attributed to several flaws in the crystal structure, such as tin interstitials, oxygen vacancies, or crystal defects.According to certain research, crystal faults expand a new energy level known as Fermi energy levels, which may be the reason why the energy gap between materials narrows.This proposes the involvement of larger particle size and the presence of the Sn-O phase, since the electronic structure and band gap of tin oxide(SnO 2 ) are significantly influenced by the particle size, the structural change and particle morphology 21 .The sample's oxygen content is further confirmed by the band gap's narrowing 19 .

FTIR analysis
The ATR-FTIR spectra of Croton macrostachyus extracts along with synthesized SnO 2 NPs extract are shown in Fig. 3. Figure 3a shows the IR spectra of plant extract which indicate the presence of bio-reduction in plant extract.The absorption pattern at 491.95 cm −1 represents the Alkyl and Aryl Halides C-I stretch are associated.
The absorption spectra at 1037.3b shows the formation of SnO 2 NPs after the reaction.Here all the functional groups present in plant extract are reduced and only the properties of the required nanoparticle are shown on the graph which indicates the formation of SnO 2 nanoparticles.The peak at 1629 cm −1 represents C=C stretching alkenes di-substituted.The absorption pattern at 506 cm −1 shows the Sn-O functional group.Generally, the interpretation of FTIR spectra from the study is well supported by the previous study in which the synthesis of SnO 2 NPs was performed using extracts of different plants 22 .

XRD observations
The XRD diffraction is one of the instruments used to discover the crystalline structure and particle size of the prepared NPs. Figure 4 shows the XRD patterns of all green synthesized SnO 2 NPs.The peaks with 2θ values at 26.83  101), ( 210), ( 211), ( 002), ( 301) and ( 222) planes, respectively, indicating the formation of SnO 2 with spherical structure.This is indexed with the JCPDS card of SnO 2 nanoparticles (File no: 041-1445).The result obtained in this study agreed with previously reported 18 .The average crystallite size was measured using Debye-Scherer's Equation 23 and the average crystallite size (D) was calculated to be 32.18 nm.Debye-Scherer's equation is given by:  where D is the crystallite size, is the wavelength, β is the full-width half maximum (FWHM) in radians, and θ is the Bragg's angle.The SnO 2 NPs crystal size calculation using Debye-Scherer's equation and data from Fig. 4 are listed in Table 2. (

Scanning electron microscope (SEM) and EDX analysis
SnO 2 nanoparticles were prepared from Croton macrostachyus leaf extract.The morphological characteristics of these nanoparticles were examined using a field emission scanning electron microscope.The morphology of SnO 2 nanoparticles in SEM images consists of very fine flakes with tiny agglomerates.The presence of these atoms can attest to the development of the pure SnO 2 phase 24 .The elements included in the sample are identified using energy-dispersive X-ray spectroscopy (EDAX), which measures the X-rays released from the sample following e-beam excitation.The process relies on an incident electron ejecting an inner shell electron from the sample, ionizing the atoms within.Utilizing EDAX spectrometers, the elemental composition of oxygen and tin was verified (Fig. 5 and Table 3).Energy is shown in kiloparsec (KeV) on the horizontal axis, while the number of X-ray counts is shown on the vertical axis.

X-ray photoelectron spectroscopy (XPS) analysis
To analyze the surface chemistry and compositions of the SnO 2 nanoparticles, X-ray Photoelectron Spectroscopy (XPS) was employed.Figure 6a displays the overall scan of the samples.Further insights into the chemical makeup are provided in Fig. 6b,c, showcasing the XPS spectra of Sn 3d and O 1 s for the prepared SnO 2 nanoparticles.Figure 6b reveals doublet peaks corresponding to Sn 3d 5/2 and Sn 3d 3/2 , providing further characterization of the tin oxidation state.In Fig. 6c, the presence of oxygen is predominantly associated with the surface of SnO 2 , suggesting its bonding with O 2 ions within the tetragonal structure of Sn 2+ ions.The non-uniform shape of the O 1 s peak suggests the existence of additional oxygen species on the surface.The XPS analysis of the Sn 3d corelevel presents a distinct doublet, suggesting the presence of Sn 4+ in the SnO 2 form 25 .Atomic percentage (%) of O 1 s and Sn 3d of the as-synthesized SnO2 NPs was found to be 83.31 and 16.86 respectively.

Dynamic light scattering (DLS) analysis
We can only estimate the appropriate crystalline size using Debye Scherrer's equation.Accordingly, one needs either utilize Dynamic Light Scattering (DLS) or transmission electron microscopy (TEM) to obtain a reliable size assessment of nanoparticles despite their size and morphologies.In this work, the DLS approach was used to determine the size distribution and average particle size of the biosynthesized SnO 2 NPs. Figure 7 illustrates how the SnO 2 NPs' particle size is generally larger than the size of the crystallite because DLS provides information about the hydrodynamic size of particles, which may differ from their actual physical size, especially for non-spherical or aggregated particles.Additionally, proper data interpretation may involve considering the sample's refractive index, viscosity, and other experimental conditions.Overall, DLS is a powerful technique for characterizing nanoscale particles and colloidal systems, providing valuable insights into their size and stability.
Reports of a similar combination employing a chemical approach also exist 10,11 .The crystal size of the different concentrations of the prepared samples were indicated in Fig. 7a-d and it is shown that the average approximate    www.nature.com/scientificreports/size of the NP is equal for different concentrations of the samples which the difference may not affect the catalytic degradation properties.

BET analysis/surface area analysis
The prepared SnO 2 NPs surface area and porosity were assessed through a Nitrogen adsorption-desorption experiment conducted at 77 K, as illustrated in Fig. 8.The surface are of the particle is calculated by using Brunauer-Emmett-Teller (BET) method and found to be 212.665m 2 /g.The BJH (Barret-Joyner-Halenda) method was used to calculate the pore volume and average pore diameter of the as-synthesized SnO 2 NPs and it is found to be 0.11 cc/g and 3.1 nm, respectively (see Fig. 8a,b).

Formation of tin oxide (SnO 2 ) nanoparticles
Green synthesis typically refers to environmentally friendly methods that aim to minimize the use of hazardous substances and energy.In this case, tin oxide (SnO 2 ) nanoparticle is formed from tin (IV) chloride pentahydrate (SnCl 4 ⋅5H 2 O), different functional groups in plant extract and deionized water.The pathway for the formation of SnO 2 nanoparticles is illustrated in Eqs. ( 3)-( 5) below: Step 1: Dissociation of SnCl 4 •5H 2 O: SnCl 4 •5H 2 O dissociates in water to release Sn 4+ ions and Cl − ions: Step 2: Hydrolysis of Sn4 + ions: The Sn 4+ ions react with water molecules to undergo hydrolysis, forming Sn(OH) 4 : Step 3: Condensation of Sn(OH) 4 : The condensation reactions involve the removal of water molecules from Sn(OH) 4 , resulting in the formation of SnO 2 nanoparticles.The process can be represented as follows: The Croton macrostchayus organic molecules work as stabilizing sources by binding to SnO 2 ions, suppressing growth.Consequently, the size of the SnO 2 NPs determines the percentage of extract that is used 17 .The size and morphology of the SnO 2 nanoparticles can be influenced by various factors, including the reaction conditions, precursor concentration, and the presence of additives or surfactants.

Rhodamine B (RhB) degradation
For degradation purposes we used the 1:4 sample ratios for both RhB and MB dyes.The sample can utilize the visible spectrum of visible light irradiation due to its 2.42 eV band gap after being calcined at 500 °C and this band gap is valuable to use in visible radiation as reported 19 .To assess the SnO 2 NPs' ability to degrade effectively under photocatalytic action, rhodamine B (RhB) was used as a contaminant.The photocatalytic activity was supposed to be time-dependent.The photocatalytic experiment was conducted in a stainless steel reactor that was closed.To ensure the solubility of the rhodamine B (RhB) dye at the beginning of the reaction, 50 mg of the photocatalyst samples, a rhodamine B (RhB) from stock solution, and 18 ml of DI water are added and stirred magnetically for 30 min in the dark before light irradiation.After irradiating the solution with visible light, 0.5 mL of the sample was withdrawn every 20 min interval and reported using UV-vis spectroscopy.
Rhodamine B (RhB) dye has distinctive absorption spectra at 554 nm, and Fig. 9a,b shows the degradation of RhB following without the addition of catalyst and with the addition of biologically made SnO 2 NPs in which the degradation is slow in 80 min shows only 7.76% and the absorption intensity steadily declined and vanished in 80 min after the addition of the prepared catalyst shows 86.12% of degradation.Rhodamine B (RhB) dye degrades in an aqueous solution is also evidenced by the visible change in color from pink-red to a colorless solution.From this result, it is concluded that the possible time required for the deprivation of rhodamine B (RhB) dye was 80 min for the prepared solution.This is related to the previous reported literature 26,27 .From the relatively few findings on the visible light photocatalytic performance of SnO 2 , the synthesized sample of SnO 2 exhibits improved results in the destruction of RhB.The percentage degradation per minute, pseudo-first-order kinetics, and half-life time of degradation were obtained by Eqs. ( 6)-( 8) shown below 28 .
where A o and A t are the initial concentration and concentration of RhB and MB at the sampling time of 't' respectively, K app is the kinetic constant and t 1/2 , the half-life time of the dye degradation.
From the obtained result, the rate constant of degradation per minute was K = 0.02689 (2.689 × 10 −2 min −1 ), the pseudo-first-order kinetics (Fig. 10) which relies at time increases lnC/Co decreases which resulted in 91.26% (Fig. 11b) of degradation in 80 min and a half life time of degradation was 25.66 min.This value agrees with previous reported literatures 29 .Percentage degradation without catalyst is illustrated in Fig. 11a.

Methylene blue degradation
The ability of SnO 2 NPs to break down Methylene blue (MB) in the presence of visible light was assessed using MB as a contaminant.The photocatalytic activity was supposed to be time-dependent.The photocatalytic experiment was conducted in a stainless steel reactor that was closed.To ensure the solubility of the Methylene blue (MB) dye at the beginning of the reaction, 50 mg of the photocatalyst samples, a Methylene blue from stock solution, and 18 ml of DI water are added and stirred magnetically for 30 min in the dark before light irradiation.After irradiating the solution with visible light, 0.5 mL of the sample was withdrawn every 20 min time and reported using UV-vis spectroscopy.
The analysis of photocatalytic degradation for the prepared SnO 2 nanoparticles is shown below for MB under visible light in Fig. 12. MB dye shows typical absorption spectra at 664 nm.The proof of the presence of degradation is indicated by a change in the UV-vis spectra in Fig. 10b.Also the degradation is observed from the color change of blue to colorless during the degradation process.From the spectral changes, the absorbance of the solution along with the time of treatment also reduced.From the graph (Fig. 14b) it is observed that the solution degraded 96.35% after 80 min of visible light exposure.This is because the nanoparticles are smaller and have a narrower band gap, which leads to a higher surface area and improved photocatalytic activity, as shown by earlier studies 17 .For the MB dye, the rate constant of degradation per minute, pseudo-first-order kinetics, and half-life time of degradation was obtained by equation below 28 .From the obtained result, the rate constant of degradation per minute was K = 0.04286 (4.286 × 10 −2 min −1 ), the pseudo-first order kinetics of degradation before and after the catalyst added (Fig. 13a,b) which relies as on time increases lnC/Co decreases which resulted in 96.35% (Fig. 14b) of degradation in 80 min and a half life time of degradation was 16.1 min.This value agrees with previous reported literatures like 30 .Percentage (%) degradation without catalyst is shown in Fig. 14a.
In general, the percentage of degradation of Methylene blue (MB) dye is faster than that of the Rhodamine B (RhB) dye for the chosen concentration in the result of the study which agrees with the previous report 31,32 .

Effect of PH on degradation
Experiments at different pH values were carried out while keeping the catalyst load and dye concentration constant in order to assess the ideal pH for the degradation of methylene blue (MB) and Rhodamine B (RhB) dyes.Protons are released in order to complete the photodegradation.The degradation of RhB and MB for pH 3, 6, 9 and 11 was tested under the irradiation time of 80 min was shown in Fig. 15a,b respectively and it is indicated that maximum PH was observed at PH 6 for both dyes.Beyond this pH, there is less degradation and a thin coating of ions with the opposite charge is drawn to the surface of the nanoparticles due to their surface charge.species in charge of the photocatalytic breakdown of RhB and MB were hydroxyl radicals ( _ OH).The same study has been reported in papers 14,39 by green and chemical method synthesis respectively for SnO 2 and TiO 2 for the photodegradation of RhB and bisphenol A under visible light which validated the result obtained in this paper.

Photo-oxidation reaction mechanism of dye degradation
The photo-oxidation degradation mechanism for tin oxide nanoparticles involves the interaction of the nanoparticles with light and oxygen, leading to changes in their structure and properties.Tin oxide (SnO 2 ) is a semiconductor material commonly used in various applications, including gas sensors, solar cells, and catalysis.When exposed to light and oxygen, tin oxide nanoparticles can undergo photo-oxidation, which is a process where the material reacts with oxygen under the influence of light.
Here is a simplified explanation of the photo-oxidation degradation mechanism for tin oxide nanoparticles: tin oxide nanoparticles absorb photons from light, especially in the UV or visible regions, leading to the excitation of electrons within the material.The absorbed photons create electron-hole pairs within the tin oxide nanoparticles.Electrons are excited to higher energy levels, leaving behind positively charged holes.The excited electrons and holes participate in redox reactions with adsorbed oxygen molecules (O 2 ) from the surrounding environment.The oxygen molecules are often adsorbed on the surface of the nanoparticles.
The detailed mechanism can vary depending on the specific conditions, but here is a simplified explanation: 1.Generation of Electron-Hole Pairs: When tin oxide is exposed to UV/Visible light, electrons in the valence band can be excited to the conduction band, creating electron-hole pairs.where hν represents the energy of absorbed light and SnO 2 * is the excited state of the NPs.

Reactive Oxygen Species (ROS) Formation:
The photogenerated holes ( h VB + ) and electrons ( e CB + ) can react with oxygen and water molecules on the surface to form reactive oxygen species, such as superoxide radicals (O 2 − ), hydroxyl radicals (OH − ), and peroxide species.

Oxidation of Organic or Inorganic species:
The generated ROS are highly reactive and can oxidize organic and inorganic species on the surface of tin oxide.This oxidation process leads to the degradation of contaminants.
The overall process involves the generation of electron-hole pairs, the formation of reactive oxygen species, and the subsequent oxidation of contaminants on the tin oxide surface.This mechanism is often exploited in the field of photocatalysis for environmental remediation and degradation of pollutants.Keep in mind that the specific reactions and intermediates involved can vary based on factors such as the type of tin oxide, the presence of co-catalysts, and the nature of the contaminants being degraded.

Conclusion
In the present study, SnO 2 NPs were produced by utilizing an eco-friendly synthesis method.The conversion of the bulk tin salt into tin oxide NPs was demonstrated by a change in hue.The FTIR result showed the existence of Sn-O bond peaks at 506 cm −1 .The band gap calculated from UV-Vis result by TAUC's relation was recommended as the catalyst for visible light photodegradation of pollutant dyes.In structural analysis, XRD results illustrated that the characteristic peaks of SnO 2 appear at angles related to the (110), ( 101), ( 210), ( 211), (002), (301), and (222) planes and the crystallite sizes were found to be 32.18 nm.The SEM picture displays agglomerate surface morphology.Our study demonstrated a notable shift from the conventional UV absorbance of SnO 2 NPs to visible absorbance through the use of Croton macrostachyus plant extract.This novel approach opens up exciting possibilities for applications in the visible light spectrum.The practical significance of achieving visible absorbance, with its potential benefits in terms of photocatalytic activities of pollutant dyes under visible light absorbance, suggests that further exploration of this alternative approach is warranted.When exposed to visible light, rhodamine B and Methylene blue dye undergo photodegradation.Bio-mediated SnO 2 displayed a maximum degradation efficiency of 86.12% after 80 min of irradiation for Rhodamine B (RhB) and showed 96.35% MB dye after 80 min.From the result, it can be concluded that the photodegradation rate is higher for Methylene Blue when compared to Rhodamine B for equal concentrations of catalyst and dye dosage.From radical scavenging activity, the study revealed that, from electrons and holes the predominant reactive species in the degradation mechanism was hydroxyl radicals ( _ OH).The results show that SnO 2 NPs have high quality photocatalytic degradation applications.The reason for its high photocatalytic degradation in the visible region for SnO 2 nanoparticles is its low band gap energy of the prepared NPs and the preparation techniques also affects it.
In general, in this work green methodology was employed to make the semiconductor oxides without the need for a particular environment, and it is an affordable technique for producing nanoparticles at low temperatures.Similar photocatalytic properties to those produced by chemical synthesis can be seen in the metal oxides produced using this approach.Our synthetic process uses metal salts solely, which are used to make metal oxides, and are safe and cheap to utilize.As a result, the semiconductor metal oxides engaged in the present work suggest a green pathway for the degrading processes.

Figure 1 .
Figure 1.Schematic diagram of Green synthesis route in SnO 2 NPs.
the x-axis to decide the energy gap values.SnO 2 NPs has a related assessed band gap values (see Table

14 )Figure 16 .
Figure 16.(a) The degradation mechanism of RhB and (b) MB over biosynthesized SnO 2 NPs in the presence of isopropanol and Na 2 EDTA radical scavengers.

Table 2 .
The SnO 2 NPs crystal size calculation using Debye-Scherer's equation and data from Fig. 4.

Table 3 .
Chemical composition of synthesized SnO 2 nanoparticles from EDX results.